1. Field of the Invention
The present invention relates to a process for producing paper materials such as paper, linerboard or corrugated linerboard from unbleached and semi-bleached chemical or semichemical pulp or pulp from recycled fibers.
2. Description of the Related Art
Linerboard and corrugated medium, used for making corrugated paperboard and corrugated cartons, are commonly made from a suspension of unbleached chemical or semichemical pulp or pulp from recycled fibers.
Typically, the pulp is treated in a screening process, refined, then mixed with paper making additives in the stock preparation section before the pulp suspension is de-watered on the paper/board machine, and the drained water (so-called white water) is recycled back into the process for dilution of the screened stock.
The white water will normally contain high amounts of wood fibers/fines, sterol esters, resin acids, lignans, and lignin fragments typically in concentrations of 100-500 ppm or higher; all of this material will have phenolic or phenol-like groups.
Strength, particularly compression strength, is an important mechanical property of the unbleached board grades used to make corrugated boxes: linerboard and corrugated linerboards. Due to new governmental rules in some countries giving an alternative specification based on combined board edge crush and since combined edge crush can be tied directly to the compression strength of the board""s components it is now possible to sell board on a performance per square meter basis rather than only by weight.
Clearly, the wet strength of unbleached board grades used to make corrugated boxes is also of importance.
EP 429,422 discloses reduction of energy consumption in the refining stages by use of laccase during pulp preparation between the first and second refining stage; the document indicates that some increase of paper strength is also obtained.
WO 93/23606 (EP 641 403) discloses a process for treating a mechanical pulp with a phenol-oxidizing enzyme system to increase the strength of the produced paper.
WO 95/09946 discloses a process for producing linerboard or corrugated medium having increased strength by treating pulp with a phenol-oxidizing enzyme.
WO 95/07604 discloses a process for producing fiberboard having improved mechanical properties by treating a slurry or suspension of a lignin-containing wood fiber material with a phenol-oxidizing enzyme.
U.S. Pat. No. 4,687,745 discloses a process for enhancing the strength properties and brightness stability of mechanical pulp by treating the pulp with ligninolytic enzymes. The wet strength of paper materials may be enhanced by adding wet strength resins to the pulp. However, these resins will enhance the strength of the paper material in such a way that re-use of the paper material will become difficult.
It is an object of the present invention to provide a process for producing, from unbleached or semi-bleached chemical or semichemical pulp, paper materials such as paper, linerboard or corrugated linerboard having improved wet strength.
The present inventors have now surprisingly found that the wet strength of paper materials can be increased by treating a pulp suspension with a phenol-oxidizing enzyme system prior to the paper machine. It has also been found that the wet strength can be further improved by a combined enzyme/mediator treatment. The wet strength may be even further improved by additionally applying a heat treatment.
Accordingly, in a first aspect the present invention relates to a process for producing paper materials with improved wet strength, comprising:
(a) preparing a suspension of unbleached or semi-bleached chemical or semichemical pulp or pulp from recycled fibers;
(b) treating the pulp with a phenol oxidizing enzyme and a mediator; and
(c) de-watering the treated pulp in a paper making machine to remove process water and produce the paper material.
In a preferred embodiment, the process water from step (c) is recycled, and step (a) comprises dilution of the pulp with the recycled process water. Advantageously, the enzymatic treatment of the pulp and white water suspension will to a large extent polymerize the aromatic materials present in the white water (lignans, resin acids, sterol esters, lignin-like compounds fibers and fines), so that they are retained in the paper sheet, leading to an increased yield and a decreased COD (chemical oxygen demand) load and toxicity of the effluent. This polymerization is also believed to contribute to strengthening of the linerboard or corrugated medium.
In a further preferred embodiment the paper material is heated after the completion of step (c).
In a second aspect the present invention relates to a process for making corrugated paperboard or corrugated boxes using the linerboard and/or corrugated linerboard produced by the process of the invention.
In a third aspect the present invention relates to the use of a phenol-oxidizing enzyme in combination with a mediator to produce a paper material with improved wet strength.